Project Summary/Abstract 22q11.2 deletion syndrome (22q11.2del) is the most com~mon human microdeletion syndrome known, affecting some 1/4000 individuals. Most patients have a 3 Mb deletion on one copy of chromosome 22q11.2, resulting in a haploinsufficiency of over 107 genes, 46 protein coding and the remainder noncoding RNAs and pseudogenes. Children born with this deletion can have a range of congenital anomalies that often include three that are developmentally linked; hypoplasia of the thymus, congenital heart defects (CHD), and hypoparathyroidism. Approximately 60-70% of the patients have an immunodeficiency due to reduced T cell output from a hypoplastic thymus and are often clinically referred to as having DiGeorge syndrome. The underlying mechanisms causing the defective formation/patterning of the thymic tissue remains poorly understood. Our results, obtained during the previous cycle of this grant, suggest a defect among the neural crest derived mesenchymal cells, which form the thymic capsule and vasculature and regulate the expansion of the thymus. In our first aim, the developmental abnormalities of the pharyngeal apparatus leading to the formation of a hypoplastic thymus will be determined. Specifically, the role of the neural crest-derived mesenchymal cells in regulating the development and expansion of the thymus will be studied. Embryonic thymii from mouse models of 22q11.2del will be used in reaggregate fetal thymic organ cultures to define the role of mesenchymal cells in the process of thymus expansion. RNA sequencing approaches, including single cell RNA sequencing will be used to determine what mesenchymal transcripts are involved in this process. These experiments will be complemented with a characterization of human thymii from 22q11.2del patients and normal controls. In humans with 22q11.2del along with the mouse models, there is a post-natal miRNA dysregulation noted. In aim 2, we will explore the consequence of these miRNA changes using a combination of longitudinal studies in humans and diverse mouse models. This will reveal whether the dysregulation of miRNAs impacts immune functions pertaining to the thymus. Results from the two aims will enable us to develop better strategies for restoring thymus functions in various clinical settings resulting in the hypoplasia of this tissue.